I have an Auto-Chinon 135mm f/2.8, in M42 mount that I bought off eBay for a princely sum of about £20. As a previous poster says, it takes practice practice practice. I ended up mostly using (M)anual mode on the camera, as although it was an f/2.8 lens, the display on my 7D only ever told me it was an f/1.4!
The metering itself came almost naturally after a while. I guess older photographers who remember the old manual cameras will get on easier than us young'uns, but on a bright day, I'd set ISO 100 or 200, and then go from there. If I was wide open at f/2.8 I'd start with a shutter speed of about 1/320th, take a picture, and check the display. Adjust as necessary. If I wanted to stop down to f/8, I might boost the ISO to 400 and drop shutter speed down to 1/125th? There is no specific guidance as such but I just used a try, view, adjust, repeat methodology. Maybe this isn't the best way to do it but it's an option.
Don't forget if you shoot to RAW, its easier in post processing to make exposure adjustments as necessary so you can reduce or increase the brightness (to a certain extent) later.
As for focussing, the M42-EF mount adapter I had, albeit a cheapo one from Hong Kong (also from eBay) had some little electronic contacts on it that seemed to make the focus confirmation light work. I imagine your 600D would have the same thing? If you look in the viewfinder, at the bottom-right, when you are focussed on a subject, a green dot appears - I used this to know if I was in focus or not.